The radio frequency (RF) band of the electromagnetic spectrum contains frequencies from approximately 3 kilohertz (3,000 hertz, or 3 kHz) to 300 gigahertz (GHz). In many places, the use of this band is regulated by the government. For example, in the United States, the broadcast television system is disseminated via radio transmissions on designated channels in the band from 54 megahertz (MHz) to 890 MHz. There are two radio frequencies at which satellites broadcast signals in the Global Positioning System (GPS); L1 signals are broadcast at 1.57542 GHz, and L2 signals at 1.2276 GHz.
Some radio receivers, such as super-heterodyne receivers, operate by converting a high RF signal to a signal of a lower frequency, often referred to as an intermediate frequency (IF) by mixing the RF signal with a mixing signal of a different frequency, to allow for more convenient amplification and selection of the desired channel. The difference between the frequencies of the RF signal and the mixing signal is the frequency of the IF signal. (As used herein, a receiver that receives any signal in the RF band is a radio receiver, even if the signal is a television or GPS signal as above.) Signals in the television spectrum may be down-converted so that, for example, an RF signal in the 500 MHz to 506 MHz region (which is TV channel 19) may be down-converted to an IF signal at 41 MHz to 47 MHz. Other television signals, or the GPS signals described above, may be similarly down-converted. Application of gain and selection of the channel to be received can thus occur in the IF frequencies, which are more easily operated upon than the higher RF frequencies.
In modern radio receivers, frequency selection and data recovery is performed by converting the down-converted IF signal into the digital domain. An analog-to-digital converter (ADC) is used to transform the analog IF signal into a digital data stream after which sophisticated digital signal processing (DSP) techniques can be used to recover from noise, dropout and similar artifacts of a digital radio system.
It is known to be desirable to move as much of the receiver into the digital domain if possible, as this would allow all receiver features such as channel selection and protocol implementation to be done digitally, at lower cost and with higher performance than is currently available. Thus, for example, the idea that the RF signal itself could be converted with either a conventional finite impulse response (FIR) filter or a high speed ADC appears to be attractive.
However, as is known in the art, to convert an analog signal to a digital signal, an ADC must sample the analog signal at a rate at least twice as fast as the signal itself. Thus, to achieve such direct conversion of an RF signal with a frequency of 1.25 GHz (including all signals of lower frequencies) would require at least 2.5 giga-samples per second (GS/s), or one sample every 400 picoseconds (pS). This would require a conventional FIR filter to pass samples from one sample-and-hold amplifier (SHA) to the next with a transit time of much less than 50 pS per stage; further, as will be understood by one of skill in the art, to allow each stage to settle to 60 db, or one part per thousand, implies a bandwidth of about 22 GHz.
Alternatively, a high speed ADC operating at a frequency of at least 2.5 GS/s would similarly be sufficient to convert a 1.25 GHz signal. Such high speed ADCs consume a large amount of power. Further, the data emerging from such an ADC is at an exceptionally high rate, since each sample must contain a desired number of bits and the total output rate is the sampling rate times the number of bits per sample. Thus, a 12 bit ADC running at 2.5 GS/s outputs about 30 gigabits per second (GB/s), a large amount of data to transport and process.
Further, as is known in the art, a conventional heterodyne receiver which down-converts by mixing a RF signal with a lower frequency signal has an “image problem.” Suppose a 500 MHz signal is down-converted to one at 5.2 MHz by mixing the 500 MHz signal with one at 494.8 MHz (the IF signal is the difference between 500 MHz and 494.8 MHz, i.e., 5.2 MHz). There will also be the down-conversion of an “image” signal; a signal having a frequency of 489.6 MHz will also be down-converted to 5.2 MHz, since just as 500 MHz less 498.8 MHz equals 5.2 MHz, so too does 494.8 MHz less 489.6 MHz equal 5.2 MHz.
Both 500 MHz and 489.6 MHz thus down-convert to 5.2 MHz, which creates a problem, since once converted the two signals cannot be separated. The second, unwanted signal is referred to as the “image,” since it includes the signal at the frequency (here 489.6 MHz) that is the same distance from the local frequency of 494.8 MHz as the desired 500 MHz signal, and thus the “mirror image” of the desired signal. To be useful, a radio must be able to provide “image rejection,” i.e., to be able to reject the unwanted image frequency.
A known method of image rejection is known as the Weaver architecture. A circuit 100 using this architecture is shown in FIG. 1; it uses two sets of multipliers, each set called a “quadrature modulator” because it multiplies the signal by a local oscillator that has two outputs that are in quadrature to each other, i.e., 90 degrees apart in phase. There are four multipliers 101, 102, 103 and 104, and two low pass filters 105 and 106.
The RF signal is divided into two paths; on one path the signal is multiplied by sine waves at a first frequency by multiplier 101 and at a second frequency by multiplier 103; in between is a low pass filter 105. On the other path, multipliers 102 and 104 multiply the signal by cosine waves at the same two frequencies, again separated by an identical low pass filter 106.
(It will be apparent to one of skill in the art that it is common to refer to a single “multiplier” that multiplies an input signal by some function, for example, a sine or cosine wave, when what is really meant is a plurality of multipliers that each receive time-separated samples of the signal and coefficients, such that the sum of the outputs of the plurality of multipliers is the input signal multiplied by the function, i.e., the desired sine or cosine wave. FIG. 1 shows such a single multiplier, as do other figures herein. One of skill in the art will understand when a multiplier being discussed is intended to be so representative of a plurality of multipliers. The sampling circuits, such as sample and hold amplifiers (SHAs), are also not shown in FIG. 1.)
The two frequencies are selected so that the net result is to down-convert the incoming RF signal by the desired amount; the first multipliers 101 and 102 down-convert the RF signal part of the way, and the partially down-converted intermediate signal is filtered. The second multipliers 103 and 104 down-convert the signal the rest of the way to a desired frequency, resulting in an IF signal. The low pass filters are selected to filter the intermediate signal so that the upper of the mixed frequencies is removed, and only the difference frequency FREC−FLO between the received frequency FREC and the local oscillator frequency FLO is passed to the second set of multipliers 103 and 104, while the sum frequency FREC+FLO is removed. Thus, there can be no image present in the IF output. Any direct conversion of an RF circuit into the digital domain must also provide for such image rejection and thus implement such a Weaver architecture or a functional equivalent digitally.
Despite the potential difficulties of sampling at such high speeds, large amounts of resulting data, and image rejection, the ability to directly sample and convert the RF signal remains very attractive. If such direct conversion could be done, and signals originally widely separated in frequency down-converted to a more narrow range, a programmable device could function as a flexible multi-purpose radio receiver controlled entirely by software, and without needing any frequency-specific elements. Such a “software defined radio” or SDR could potentially operate on multiple channels at the same time. For example, in such a case the receiver could decode digital TV on channel 19, while at the same time decoding the GPS position and receiving digital data from a 2.4 Ghz signal.